1) Field of the Invention
The present invention relates to an amplifier used in semiconductor integrated circuits.
2) Description of the Related Art
FIG. 8 is a circuit diagram that shows a configuration example of an amplifier used in conventional semiconductor integrated circuits. The amplifier shown in FIG. 8 includes two current mirror circuits 803 and 804. The current mirror circuit 803 includes PMOS transistors 831 and 832. Source electrodes of the PMOS transistors 831 and 832 are connected to a power supply 801. The current mirror circuit 804 includes NMOS transistors 841 and 842. Source electrodes of the NMOS transistors 841 and 842 are connected to ground (GND) 802.
The diode-connected PMOS transistor 831 is connected at its drain electrode to a first terminal of a resistor 806. The diode-connected NMOS transistor 841 is connected at its drain electrode to a first terminal of a resistor 807. A second terminal of the resistor 806 and a second terminal of the resistor 807 are directly connected to form a node A. An input signal source 805 is connected to the node A and ground (GND).
Drain electrodes of the PMOS transistor 832 and the NMOS transistor 842 are directly connected to each other, and led to an output terminal, which is not shown. A series circuit consisting of a load resistor 808 and an output bias voltage source 809 is connected between a node of the drain electrodes of the PMOS transistor 832 and the NMOS transistor 842 and the ground (GND) 802.
The configuration is designed so as to cause the current mirror circuits 803 and 804 to provide the same current ratio. Resistors 806 and 807 are equal in resistance. Supposing that a voltage at the power supply 801 is Vcc, a voltage at the node A is Vcc/2. Therefore, the input signal source 805 has a voltage of Vcc/2 when there are no signals.
When the input signal source 805 has no signals, the two current mirror circuits 803 and 804 balance with each other and a current that flows out from the drain electrode of the PMOS transistor 832 becomes equal to a current that flows into the drain electrode of the NMOS transistor 842. At this time, a current does not flow through the load resistor 808. An output.voltage in this state is a voltage at the output bias voltage source 809.
If the voltage at the input signal source 805 becomes lower than the voltage at the node A, then the balance between the current mirror circuits 803 and 804 is lost. The current that flows out from the drain electrode of the PMOS transistor 832 becomes larger than the current that flows into the drain electrode of the NMOS transistor 842. A current that corresponds to the difference flows through the load resistor 808. Therefore, the output voltage rises above the voltage at the output bias voltage source 809.
On the contrary, also when the output voltage at the input signal source 805 becomes higher than the voltage at the node A, the balance between the current mirror circuits 803 and 804 is lost. In this instance, however, the current that flows out from the drain electrode of the PMOS transistor 832 becomes smaller than the current that flows into the drain electrode of the NMOS transistor 842. Therefore, a current flows from the output bias voltage source 809 into the NMOS transistor 842 through the load resistor 808. As a result, the output voltage falls below the voltage at the output bias voltage source 809.
In this way, the conventional amplifier is formed as an inverting amplifier, in which the output voltage changes in an opposite direction according to the input signal voltage.
With the advance of a finer pitch of the semiconductor integrated circuit design rule, the operation power supply voltage is becoming lower. A lower operation power supply voltage brings about an advantage that the power consumption is reduced. On the other hand, the lower operation power supply voltage brings about an evil influence that a wide dynamic range cannot be obtained especially in analog circuits. Therefore, it is desirable to use substantially the whole range between the ground potential and the power supply voltage as the operation range in an amplifier that operates with a low voltage.
In the conventional amplifier having the configuration, however, the voltage at the input signal source that is in the vicinity of the power supply voltage or the ground potential causes circuit saturation in the current mirror circuits 803 and 804, and obstructs normal operation of the current mirror circuits. Therefore, it is difficult to widen the dynamic range.
The gain of the amplifier is determined according to current values of the current mirror circuits 803 and 804, and also to the resistance of the load resistor 808. The current values of the current mirror circuits 803 and 804 are determined by resistances of the resistors 806 and 807, and gate-source voltages of the diode-connected PMOS transistor 831 and NMOS transistor 841. Therefore, there is also a problem that the gain is affected by dispersion factors.
It is an object of this invention to provide an amplifier suitable for low voltage operation, in which a dynamic range can be expanded to the whole range between the ground potential and the power supply voltage and the gain can be prevented from being affected by dispersion factors.
The amplifier according to one aspect of this invention includes a first voltage follower circuit that includes P-type MOS transistors and outputs a current having a magnitude equivalent to a value obtained by dividing a difference voltage between an input bias voltage applied to a positive-phase input terminal and an input signal voltage applied to a negative-phase input terminal supplied directly with a feedback from an output terminal via a first resistor, by resistance of the first resistor. The amplifier also includes a first current mirror circuit that takes out the output current of the first voltage follower circuit. The amplifier further includes a second voltage follower circuit that includes N-type MOS transistors and outputs a current having a magnitude equivalent to a value obtained by dividing a difference voltage between an input bias voltage applied to a positive-phase input terminal and the input signal voltage applied to a negative-phase input terminal supplied directly with a feedback from an output terminal via a second resistor, by resistance of the first resistor. The amplifier further includes a second current mirror circuit that takes out the output current of the second voltage follower circuit.
The amplifier according to another aspect of this invention includes a voltage follower circuit that includes P-type MOS transistors and outputs a current having a magnitude equivalent to a value obtained by dividing a difference voltage between an input bias voltage applied to a positive-phase input terminal and an input signal voltage applied to a negative-phase input terminal supplied directly with a feedback from an output terminal via a resistor, by resistance of the resistor, and a current mirror circuit that takes out the output current of the voltage follower circuit.
The amplifier according to still another aspect of this invention includes a voltage follower circuit that includes N-type MOS transistors and outputs a current having a magnitude equivalent to a value obtained by dividing a difference voltage between an input bias voltage applied to a positive-phase input terminal and an input signal voltage applied to a negative-phase input terminal supplied directly with a feedback from an output terminal via a resistor, by resistance of the resistor, and a current mirror circuit that takes out the output current of the voltage follower circuit.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.